1. Field of the Invention
The present invention relates to a scanning optical system which is for use in a laser beam printer or a digital copying apparatus, and causes a light beam emitted from a light source to scan a scanning surface, and also relates to a digital beam forming apparatus incorporating the scanning optical system.
2. Description of the Related Art
A scanning optical system used in conventional image forming apparatuses is designed so that a light beam emitted from a light source is guided to a deflector through an incident optical unit, and the light beam deflected from the deflector is focused into a spot while being scanned on a scanning surface through a scanning optical unit.
The scanning optical system includes one or more optical elements. When these optical elements are decentered, an actual focused position of an image become shifted from a correct focused position. For example, a resulting image is bent in a sub scan direction, thereby degrading printed image.
A plurality of light source elements may be employed in a scanning optical system. For example, Japanese Unexamined Patent Application Publication No. 9-54263 discloses a multi-beam laser chip as a light source which emits a plurality of laser light beams arranged in a single line. When the optical elements are decentered, not only the focused position is generally deviated, but also the relative positional relationship between focused positions of light source elements is changed. The quality of printed image is adversely affected more when a plurality of light source elements are used than when a single light source is used.
To reduce image degradation, tolerances of the optical element itself and tolerances of the mounting position of the optical element during manufacture need to be reduced. Such an attempt is subject to limitation. Even if tolerances are reduced, manufacturing such a scanning optical element is difficult. The manufacturing yield thereof drops, leading cost increases.
Lenses are preferably shaped in a configuration that is less subject to the effect of decentering. Specifically, each lens preferably has a rotationally symmetrical optical surface. Such an optical surface does not affect print quality at all even when the optical surface is decentered from an optical axis. Even when the optical surface rotates about an axis parallel to the main scan direction, a deviation of a focused position with respect to the sub scan direction is substantially constant over an entire scanning range. Compared with the rotationally asymmetrical optical surface, print quality is less subject to degradation.
Several scanning optical systems have been proposed which use an optically aspherical surface or a rotationally symmetrical, aspherical surface as an optical surface in a scanning optical unit. These proposals are proposed on the assumption that the material in use is glass. An optical element constituting a scanning optical unit is typically larger in size than those used in other devices. If the optical element is manufactured of optical glass, manufacturing costs thereof is likely to increase. The use of a resin material is preferred from the standpoint of costs. Since the optical element manufactured of a resin is typically produced using a molding technique, a further cost reduction is expected because optical glass requires cutting and polishing processes. Since the resin optical element is shaped into a complex configuration, which could not be applied to optical glass, the count of optical elements is reduced. This further reduces costs of the scanning optical unit.
When an optical element is manufactured using a molding technique, a mirror surface of a die is typically corrected to set the shape of the optical surface of the optical element to designed values, and to planarize an image plane. When the optical element is pulled out from the die, the optical element suffers from a deformation such as a shrinkage because of a temperature difference between an ambient air and the die. The mirror surface of the die needs correcting to offset the deformation. If the outline of the optical element is rectangular or nearly rectangular rather than being round, the lens suffers from a difference in shrinkage rate between the long axis and the short axis. When the optical surface of the optical element is formed to be strictly rotationally symmetrical, the mirror surface of the die needs machining with the rotationally symmetrical structure anamorphically corrected. While the rotationally symmetrical shape is defined in the rotating coordinate system, the anamorphical structure is defined in the rectangular coordinate system. Both structures are thus expressed using different coordinate systems. It is thus difficult to determine a desired configuration to be applied to the mirror surface of the die. Even if such a configuration is determined, a resulting equation thereof is expected to be abstruse. Eve if the equation is implemented in correction, an error may be introduced in the actual machining.
The optical elements used in the scanning optical unit are typically rectangular or generally rectangular, and for this reason, introducing a rotationally symmetrical shape into the optical element is rather difficult.
Too much dependency on the rotationally symmetrical optical surface in design increases design limitations, thereby reducing performance of the scanning optical system, in contrast with the case in which an anamorphical surface is introduced. The number of lenses sometimes need to be increased to obtain required performance. Such a situation is undesirable from the standpoint of costs. Particularly, when there are a plurality of light source elements, a degradation in print quality results because the positional relationship between focused positions changes during scanning. High standards are required of the uniformity of sub scan magnification and the flatness of the curvature of field, compared with the case in which a single light source is used. When the sub scan magnification is not uniform over an entire scanning range, scanning lines are spaced apart more or less in the sub scan direction depending on a scan position. When the curvature of field is not flat, the positional relationship in the main scan direction is destroyed during scanning even if the spots of the light source elements are aligned at the start of writing. Print quality is thus degraded in the course of scanning when the focused positions of the light beams of the light source elements shift.
Accordingly, it is an object of the present invention to provide a low-cost scanning optical system which is made less subject to degradation in print quality by reducing the effect of the decentering of an optical element. It is also an object of the present invention to provide an image forming apparatus incorporating the scanning optical system.
The present invention relates to a scanning optical system and includes at least one light source, incident optical unit for guiding a light beam from the light source to deflector, the deflector for deflecting the light beam, a scanning optical unit, having at least one optical element, for scanning and focusing the light beam, deflected by the deflector, on a scanning surface. At least one surface of the optical element forming the scanning optical unit is an anamorphically aspherical surface in a main scan sectional plane. The relationship of 0.8 less than Ra/Rs less than 1.2 holds over an entire scanning range, where Ra represents a radius of curvature in a sub scan sectional plane of a surface which is anamorphically aspherical in a main scan sectional plane, and Rs represents a radius of curvature in a sub scan sectional plane of a surface that results if a generatrix of the surface, which is anamorphically aspherical in the main scan sectional plane, is rotated about an optical axis by 360 degrees.
Preferably, the radius of curvature Ra in the sub scan direction within a distance of 15 mm in a main scan direction from the optical axis satisfies the condition of 0.95 less than Ra/Rs less than 1.05.
The optical element having an anamorphically aspherical surface in the main scan sectional plane may be produced through a molding technique.
The optical element having an anamorphically aspherical surface in the main scan sectional plane may be a lens.
The lens having an anamorphically aspherical surface in the main scan sectional plane may be fabricated of a resin material.
The anamorphically aspherical surface in the main scan plane may be a surface of the lens of the scanning optical unit facing the deflector.
The scanning optical unit may include a plurality of lenses, at least one of which has the anamorphically aspherical surface in the main scan sectional plane.
The one of the lenses constituting the scanning optical unit and closet to the deflector preferably has the anamorphically aspherical surface in the main scan sectional plane.
The light source preferably includes one or more light source elements.
The present invention in another aspect relates to an image forming apparatus and includes the scanning optical system, a photoconductive body arranged on a scanning surface, a development unit which develops, into a toner image, an electrostatic latent image which is formed on the photoconductive body through a light beam scanned through the scanning optical system, a transfer unit which transfers the developed toner image to a sheet member, and a fixing unit which fixes the transferred image onto the sheet member.
The present invention in yet another aspect relates to an image forming apparatus and includes the scanning optical system, and a printer controller which converts code data received from an external device into an image signal and then inputs the picture signal to the scanning optical system.
In accordance with the present invention, a curvature radius Ra in a sub scan sectional plane and a curvature radius Rs corresponding to a radius of curvature in a main scan sectional plane are not equal to each other. But if the difference between the two radii is too large, the effect of the decentered optical element becomes pronounced on the scanning surface. To reduce that effect, the difference is controlled to within a predetermined value. Since the curvature radius Ra in the sub scan sectional plane and the curvature radius Rs corresponding to the radius of curvature in the main scan sectional plane are not equal to each other, design flexibility is increased, and performance of the scanning optical system is improved. Lens design thus becomes easier.
Further objects, features, and advantages of the present invention will be apparent from the following description of the preferred embodiments with reference to the attached drawings.